Add null check for prefab editor

Scripts/SoftMask.cs

@@ -339,10 +339,13 @@ namespace Coffee.UIExtensions
 		static void UpdateMaskTextures()
 		{
 			foreach (var sm in s_ActiveSoftMasks)
+			{
+				if(sm)
 				{
 					sm.UpdateMaskTexture ();
 				}
 			}
+		}
 
 		/// <summary>
 		/// Update the mask texture.

Scripts/SoftMaskable.cs

@@ -157,6 +157,8 @@ namespace Coffee.UIExtensions
 			Matrix4x4 prj = cam.projectionMatrix;
 		
 			foreach (var sm in s_ActiveSoftMaskables)
+			{
+				if(sm)
 				{
 					Material mat = sm._maskMaterial;
 					if (mat)
@@ -166,6 +168,7 @@ namespace Coffee.UIExtensions
 					}
 				}
 			}
+		}
 
 		/// <summary>
 		/// This function is called when the script is loaded or a value is changed in the inspector (Called in the editor only).